1. Field of the Invention
The present invention relates to a Cat-PECVD method, a film forming apparatus for implementing the method, a film formed by use of the method, and a device manufactured using the film. In particular, the present invention relates to a technique capable of forming high quality Si-based thin films, which are used in photoelectric conversion devices as typified by Si-based thin film solar cells, at high deposition rate over large area with uniform film thickness and homogeneous film quality.
2. Description of the Related Art
High-quality, high-deposition rate film forming techniques are crucial for improvement in performance and cost reduction of various thin film devices. In particular, for Si-based thin film solar cells that are the typical of photoelectric conversion devices, large-area film formation is also required in addition to high-quality, high-deposition rate formation of Si-based films.
Meanwhile, to classify broadly, there have been two methods known as low temperature film forming techniques: the PECVD (plasma-enhanced chemical vapor deposition) method and the Cat-CVD (catalytic chemical vapor deposition) method (the HW (hot wire)-CVD method follows the same principles). For the both techniques, research and development work has been intensively continuing focusing on the formation of hydrogenated amorphous silicon films and crystalline silicon films including micro-crystalline, mono-crystalline and poly-crystalline silicon films. Hereinafter, “crystalline silicon” is referred to silicon including micro-crystalline, mono-crystalline and poly-crystalline silicons. FIG. 4 illustrates a PECVD apparatus as conventional art 1, and FIG. 5 illustrates a Cat-CVD apparatus as conventional art 2.
In FIG. 4, there are shown a showerhead 400, a gas introduction port 401, gas effusion ports 402, a plasma space 403, an electrode 404 for plasma generation, a radio frequency power supply 405, a substrate 406, a substrate heater 407, and a vacuum pump for exhausting gas 408.
In FIG. 5, there are shown a showerhead 500, a gas introduction port 501, gas effusion ports 502, an active gas space 503, a thermal catalysis body (catalyzer) 504, an electric power source 505 for heating the thermal catalysis body, a substrate 506, a substrate heater 507, and a vacuum pump for exhausting gas 508.
To take a case where a Si film is formed using SiH4 gas and H2 gas as an example, in the PECVD apparatus shown in FIG. 4, the gases introduced from the gas introduction port 401 provided at the showerhead 400 are directed through the gas effusion ports 402 into the plasma space 403, where the gases are excited and activated to yield a decomposed species, which is deposited on the opposed substrate 406 to form a Si film. Here, the plasma is generated by means of the radio frequency power supply 405.
On the other hand, in the Cat-CVD apparatus shown in FIG. 5, the gases introduced from the gas introduction port 501 provided in the showerhead 500 are directed through the gas effusion ports 502 into the film deposition space, where the gases are activated by the thermal catalysis body 504 provided in the space, thereby to yield a decomposed species, which is deposited on the opposed substrate 506 to form a Si film. Here, the heating of the thermal catalysis body is accomplished by means of the heating power source 505.
However, these conventional techniques have the following problems:
In order to achieve high-deposition rate film formation by the PECVD method, it is necessary to promote the decomposition of the SiH4 gas and H2 gas by increasing the plasma power. However, increase of the plasma power on the other hand leads to increase in ion bombardment on the surface for deposition and promotes generation of higher-order silane species that leads to formation of powder. For this reason, this method cannot avoid incurring adverse factors that hinder the improvement of the quality.
Here, instead of increasing the plasma power, when the plasma excitation frequency is set to be in the VHF band or higher, the bombardment of ions is reduced because of the reduction of the plasma potential. This is effective for the formation of high-quality hydrogenated amorphous silicon films and crystalline silicon films. (Refer to J. Meier et al, Technical digest of 11th PVSEC (1999) p. 221, O. Vetterl et al, Technical digest of 11th PVSEC (1999) p. 233.) However, sincethe formation of crystalline Si films requires sufficient production of atomic hydrogen, increasing the plasma power is inevitable for film formation at a growth rate higher than a certain level, even if VHF band frequencies are used. Accordingly, the above mentioned problems are still unavoidable in such a case.
Also, increasing the hydrogen dilution rate, namely the gas flow ratio (H2/SiH4), may be considered as a measure for increasing the density of atomic hydrogen without increasing the plasma power. However, this causes the partial pressure of the SiH4 gas to decrease, which works contrary to the high-speed deposition. Therefore, also in this case, it is after all necessary to increase the plasma power so as to promote decomposition of SiH4. The problems mentioned above are therefore still unavoidable.
Meanwhile, increasing the pressure for film deposition may be considered as a measure for reducing the ion bombardment while allowing the plasma power to increase. However, in such a case, the reaction to generate higher-order silane species is accelerated, thereby failing to avoid factors deteriorating the film quality such as formation of powder.
On the other hand, in the Cat-CVD method, because of the nonuse of plasma, the aforementioned problem of ion bombardment does not arise in principle, and the formation of powder is minimal. Moreover, since the generation of atomic hydrogen is greatly accelerated in this method, the formation of crystalline Si films can be accomplished relatively easily and speedily. In addition, since there is no restriction in principle in enlarging the deposition area, this method has been attracting growing attention. (H. Matsumura, Jpn. J. Appl. Phys. 37 (1998) 3175–3187, R. E. I. Schropp et al, Technical digest of 11th PVSEC (1999)p. 929–930)
However, under the present circumstances, temperature increase in the substrate due to radiation from the thermal catalysis body is unavoidable. Therefore, stable formation of high quality films is not necessarily easy. In addition, since SiH4 gas is decomposed directly by the thermal catalysis body, atomic Si is inevitably generated. The atomic Si is unfavorable for formation of high quality Si films. Also, radicals such as SiH and SiH2, which are resulted from the reaction of the atomic Si with H and H2 in gas-phase, are unfavorable for formation of high quality Si films. Accordingly, it has been extremely difficult to form high-quality crystalline Si films.